Flourosilicone heat cured rubber compositions of the general purpose type are found to provide good processability and improved resistance to vapor and liquid permeability for various solvents. However, fluorosilicones have at best only an adequate physical property profile. That is, these heat cured rubber compositions possess good elongation and hardness but are deficient in other physical properties such as compression set, tensile, and resilience. These deficiencies are partially the result of the type of reactive groups in the components of the flourosilicone rubber compositions which are usually silanol functional. These compositions are usually extended with additional fillers, process aids or other agents to reduce either the out-of-pocket costs or to optimize a particular property.
Vinyl functional groups may be used that will provide a higher state of cure (degree of cross-linking as measured by Monsanto Rheometer) that yields better compression and tensile. However, these improvements are at the cost of lower elongation and resilience. The reason for this problem stems from the bulkiness of the perfluoroalkyl groups in the fluorosilicone compositions. In order to overcome steric hinderence, the use of a high weight percent vinyl is required in order to achieve reasonable reaction rates. This high percentage of vinyl, however, causes a high degree of crosslinking and a worsening of the physical properties noted above.
It has been discovered that new fluorosilicone high molecular weight (HMW) polymers having vinyl groups that are distributed randomly along the chain, as well as on some of the chain ends, 10 provide a high state-of-cure even though the total net amount of vinyl unsaturation is lower than in current polymers. The utilization of these new HMW polymers in conjunction with treated fumed silica yields polymer compounds with superior processability including lower tackiness, higher extrusion rate, better physical appearance, etc., and improved elastic response when cured.
The ability to obtain a high state of cure with lower vinyl concentration is achieved by the use of chain modifiers that eliminate or reduce steric hinderance on the chain ends from the bulky perfluoroalkyl groups. This reduction in steric hinderance on the chain ends allows vinyl-on-chain-end to crosslink easier. This reduction in steric hinderance also means less vinyl-on-chain is needed to achieve the same state of final cure which translates to less overall vinyl unsaturation.
Thus, it is an object of this invention to provide novel fluorosilicone rubber compositions which incorporate chain modifiers to reduce steric hinderance on chain ends to facilitate crosslinking.
It is also an object of this invention to provide a novel process for manufacturing fluorosilicone rubber compositions which incorporate chain modifiers to reduce steric hinderance on chain ends.